Mobile machines, including wheel loaders, haul trucks, motor graders, and other types of heavy equipment, are used for a variety of tasks. In order to accomplish these tasks, the machines typically include hydraulic and/or electric actuators that cause the machine to steer. For example, a machine may include an articulated joint and one or more associated hydraulic cylinders connected between the joint and a frame of the machine. In response to an operator input to a control system, the hydraulic cylinders expand and retract to cause a forward end of the machine to pivot about the articulated joint relative to a back end of the machine, thereby steering the machine. In another example, the machine may include one or more wheels that pivot at a vertical joint between the wheel and the frame. One or more hydraulic cylinders may be connected between the wheel and the frame of the machine to expand and retract in response to the operator input to a control system, thereby causing the wheel to pivot about the joint and steer the machine.
The steering of the machine may be controlled through a number of different strategies. One strategy includes controlling a steering position of the machine based on a positional input of the operator. In other words, as the operator turns a steering wheel or tilts a joystick lever a particular angle away from a neutral position, the articulated joint or wheel is pivoted an amount corresponding to the particular angle. Another strategy includes controlling a steering speed of the machine based on a speed input of the operator. That is, as the operator manipulates the steering wheel or joystick lever, a speed of the manipulation may be determined and the pivot speed of the articulation joint or wheel correspondingly controlled.
Regardless of which strategy is utilized to effect steering of the machine, it may be beneficial to modify the steering gain (i.e., the ratio of steering output actuation to steering input) and/or the steering deadzone (i.e., the minimum displacement amount away from the neutral position or the minimum input speed of the steering wheel or joystick required before steering of the machine is effected) in response to a travel speed of the machine. For example, when moving at low speeds, the operator may want the gain to be high and the deadzone small for responsive steering. In contrast, when moving a higher speeds, the operator may desired the gain to be smaller and the deadzone larger to prevent unintentional steering and minimize over corrections.
Examples of a positional input steering system and a velocity input steering system utilizing variable gain is disclosed in U.S. Patent Publication No. 2005/0139412 (the '412 publication) by Vigholm published on Jun. 30, 2005. The '412 publication describes a wheel loader having an articulation point and being steered by adjustment of an angle between two drive axles relative to the articulation point. A steering wheel and a steering lever are provided in parallel for steering control of the wheel loader. When either the steering wheel or steering lever are moved from a neutral position, the position of the steering wheel and lever are sensed. Based on the sensed position, the angle between the drive axles is correspondingly adjusted.
The magnitude of the angle between the drive axles described in the '412 publication and the speed at which the angle is adjusted depend on a travel speed of the wheel loader. That is, when the wheel loader is traveling relatively slow, upon moving either the steering wheel or lever to a particular position, the corresponding adjustment angle is greater and is achieved more slowly than when the wheel loader is traveling faster. In this manner, a large gentle-feeling steering deflection is provided to the operator at slow speeds, while a quick, yet controllable deflection is provided to the operator at high speeds.
The '412 publication also describes that the speed at which the adjustment angle is achieved can be affected by the steering speed input of the operator. In other words, if an operator turns the steering wheel quickly to a predetermined position, the corresponding adjustment angle will be achieved quicker than if the operator turns the steering wheel slowly.
Although the wheel loader steering system of the '412 publication may describe two alternate steering strategies (i.e., position input-position controlled and velocity input-velocity controlled), it may be limited. In particular, the system of the '412 publication does not allow the two alternate strategies to be selectively implemented within the same machine or with the same steering input device (i.e., the steering wheel or lever). There may be situations when an operator desires the velocity input of the steering wheel or lever to effect the adjustment angle such as when traveling at high speeds, and other situations when the operator desires the velocity input to have no affect such as when traveling at low speeds. In addition, when both the steering wheel and lever are provided on the same machine, an operator may desire the steering wheel input velocity to affect the adjustment angle of the wheel loader, but the steering lever velocity to have no affect on the adjustment angle. The wheel loader steering system of the '412 publication does not provide the operator with these capabilities. In addition, the '412 publication does not describe a deadzone, or a travel speed of the wheel loader affecting the deadzone.
The disclosed steering system is directed to overcoming one or more of the problems set forth above.